Senior Member

Hello bc, according to this site (johnderbyshire.com) it's a reference to Walter de la Mare. It wouldn't otherwise be understood in this context, it seems to me.
[Ed: it seems to be a phrase peculiar to Kingsley Amis; in a short space of time I have found "an old shag" used in four different pieces by him.]

and is the only writer so cited. Thus, I think that diego is partly right - Amis is using it as a disparaging term in much the same way as others use 'old bugger'. It has the advantage that 'old shag' fits the metre, whereas 'old bugger' does not.

Senior Member

I agree with pan; 'rascals' or 'lowlifes' would seem to be the meaning.

I really doubt the sexual meaning, except in the extended sense of "Don't listen to the old fuckers." The term really has virtually no sexual meaning in some contexts, and I suspect that's true of 'shag,' here.

Senior Member

diego, please stop guessing. There are several different meanings of the noun 'shag', of which two, either alone or combined, provide an appropriate meaning here. The author is Kingsley Amis, and this is a disrespectful reference to a dead poet which can be taken to mean 'old bugger', 'old goat', 'old bloke', 'old fool' or any other typically British expression of disparagement. There's no need to start searching through adjectival meanings of 'shag'.